• PDF / 2,799,413 Bytes
• 6 Pages / 414.72 x 648 pts Page_size
• 99 Downloads / 202 Views

DOWNLOAD

REPORT

---

ABSTRACT Recently, it was demonstrated that grain refinement in metals can take place through two mechanisms, namely, dynamic nucleation and remelting of initially formed dendrites. In this study, it was found that Ni99.45B0 .5 5 undergoes grain refinement, both by dynamic nucleation or by remelting, depending on the initial bulk undercooling just before crystallization. The nature of the grain refinement process is confirmed by microstructural analysis of the undercooled specimens. INTRODUCTION When a melt is undercooled below its thermodynamic equilibrium temperature T, (TI stands for the thermodynamic melting temperature of pure metals and the liquidus for alloys) and crystallizes at a temperature T, i.e., T is the kinetic crystallization temperature, the microstructure of the undercooled specimen depends on the undercooling defined as AT = T, - T. For instance, Walker [1] found that when Ni crystallizes deep in the undercooling regime, the grain size undergoes a sharp drop, by as much as two orders of magnitude, in a narrow undercooling range. This phenomenon is known as grain refinement and the undercooling which is midway in the transition range is defined as the critical undercooling for grain refinement AT*. Later, it was found that grain refinement also occurs in alloys [2] and semiconductors [3, 4]. Over the years,

many models have been proposed to explain this phenomenon, which include dynamic nucleation [5], remelting [6], and fragmentation of fragile dendrite brought about by interdendritic fluid flow [7], and surface tension driven remelting of long dendrites [8]. Recently, Leung et al. [9] demonstrated that grain refinement in undercooled Ni is brought about by shock waves or dynamic nucleation. The origin of shock waves is cavitation: Since molten Ni contracts on solidification, when the initially formed dendrites grow into the undercooled melt, the growth velocity is so rapid that the undercooled liquid is torn apart resulting in cavity formation. On the other hand, Xiao et al. [10, 11] found that grain refinement in undercooled Cu 30Ni7 0 occurs through a re-melting of the initially formed dendrites. The most important feature of an initially formed dendrite is that its Ni concentration increases from the axis to the outer boundary, thus the melting temperature of the dendrite is non-uniform, increasing as one moves away from the axis. This structure is unstable against melting. The detailed break-up mechanism of the dendrite is described in Ref. [11]. We report here experimental evidence, mainly through microstructural studies, of the two grain refinement processes occurring in undercooled Ni 99.45B0 .55.

15 Mat. Res. Soc. Symp. Proc. Vol. 481 01998 Materials Research Society

EXPERIMENT Ni 9 9 .45 B 0. 55 ingots were prepared from elemental Ni (99.999% pure) and B (99.999% pure) granules. Alloying was brought about by rf induction heating under Ar atmosphere after the right proportion of Ni and B had been weighed. It has been demonstrated that by a fluxing technique, molten metals can be unde